You would need to have a redesign of all ships, since they are just not able (physically) to hold a DPX inside.
DBX as it is, is already to big. It's even tight to fit a fighter in it, well ok, you could say, there is a 2 bay option, but still, the hatch is not big enough to get a DBX in or out.
This was discussed so much times and I'm sure, that this feature will never come.
By the way, it is not a limitation by the engine but by ship design.
Also as mentioned, the coding is already done in ED, since you already can use fighter and land vehicles. So you can leave your ship on planet surface, load the SRV and you are free to go. Same mechanic as the desired feature, but the "mother" ships are just not designed to have a child the sice of a DBX or even Sidewinder.